Changes of free amino acids in leaf sap of trees subjected to flooding and drought stresses.

Few data on amino acid presence and composition in xylem sap of trees have been reported; the studies were mainly focused on the seasonal variation and the phenological stages of the trees. The aim of the present work has been to study the influence of water stresses, either drought and flooding, during the vegetative stage of walnut tree on the composition of amino acids in leaf sap. During the all vegetative season, July-September, in control trees well watered and with well drained soil, the amino acids composition remain without significant difference. In the leaf sap the concentration of amino acids ranged between 130-200 muM and it is 0.3 % only of total free amino acids in leaves. Glutamine account for about the 40-50% of the total amino acids. Glutamine has been reported to be the dominant one in the xylem of willow, poplar and black locust trees. During stress condition increase in concentration to the mM level for drought trees and over 10 mM for flooded one is the first evident effect. Glutamine remains one of the main component but the % increase in aspartate and glutamate in drought and of GABA in soil hypoxia stress are the main changes. After removing the stresses condition, the citrulline become one of main amino acids. The increase in leaf sap of free amino acids during the stress may be consequence of different causes: protein hydrolysis, synthesis or conversion. However, considering that few amino acids accounting for over 80% of the total, or more simple the release of free amino acids present in the cells, can occur.